Study On Petrological Characteristics, Genetic Interpretation And Sedimentary Environment Of Microbial Dolomite In The Dengying Formation, Yangba Section, Nanjiang County

During the long history of the earth,microbialites,as the geological records of primitive life activities,are widely distributed and have a long time span.They can be traced back to the Paleo-Archaean at the earliest time.The sedimentary period of Dengying Formation in Sinian and its period before and after are the transition period from protozoa to metazoa.The study of microbialites types and their evolution characteristics during this period will be helpful to explore the activity and evolution process of early Earth organisms.There are abundant microbialites in Dengying Formation of Sinian in Yangba Section of Nanjiang County,Sichuan Basin,and there is a lack of interference of metazoan organisms in the process of its formation and preservation,which provides rare materials for the petrological characteristics and genesis of microbial dolomite.Detailed petrological studies show that there are three types of microbialites in the Sinian Dengying Formation of Yangba Section,Nanjiang County,Sichuan Basin:stromatolites,thrombolites and composite microbialites.The stromatolites are mainly composed of planar to undulose laminaes with interfacial distribution of light and dark layers.The development of dark layers in space can be divided into distinct laminated,diffuse laminated and distinct-diffuse laminated.According to the clot structure and the occurrence of cavity,the mesoscale thrombolites are divided into three parts:dispersed,layer-like and framed.Composite microbialites with intermingled laminated and clotted textures at a sub-centimetre scaleand,which indicats that the two structures can coexist in the same local sedimentary environment.According to the different coexistence forms of the two structures,the mesoscale structures of the composite microbialites are divided into parallel and wrap around ones.The results of fluorescence analysis showed that the dark lamina in stromatolites had mechanical origin.The three subunits of dark laminae of stromatolites indicate that there are differences in the natural degradation and preservation of diagenesis of microbial mats.The clotting structure of thrombolites is formed by the natural growth of microbial communities.No dissolution was found in the clot structure,indicating that the cavities between the clot structures are primary cavities filled with seawater cement of multiple generations and have certain anti-compaction ability.The co-existence of two microbial structures in the complex microbiolites in the same local sedimentary environment indicates that different types of microbiolites are the basis for the formation of different types of microbiolites,rather than just considering environmental factors.However,changes in environmental conditions also have a great impact on the types of microbiolites.In the microbiolites formation environment,the change of hydrodynamic energy is the main controlling factor for the evolution of microbiolites type and structure combination mode.The depth,sediment flux,nutrient content and light intensity of water are also the influencing factors of microbial dominant community.According to the microbialites structure assemblage model in the study area,it is concluded that stromatolites generally occur on the top of the thrombolites.Within the thrombolites,the dispersed thrombolites is the lowest layer,and both layer-like and framed thrombolites occur on the top of the dispersed thrombolites.Based on the analysis of microbial petrology and structural genesis of Dengying Formation of Sinian in the study area,it is considered that the second and fourth members of Dengying Formation in the study area are generally tidal flat subfacies under limited platform facies,and the sedimentary water environment is locally changed but relatively stable as a whole.Generally,it is a warm and suitable clean water environment with abundant illumination and flourishing natural development of microbial community.Finally,the sedimentary model of microbialites in the study area is established:laminar stromatolites mainly grow in low-energy environments with weak hydrodynamic action,mainly in the lower part of the supratidal zone to the upper part of the intertidal zone.Compared with laminated stromatolites,the thrombolites are more likely to grow in a hydrodynamic environment,mainly in the lower part of the subtidal zone to the lower part of the intertidal zone.Because of the coexistence of laminar structure and clotted structure,the living environment of composite microbialites should belong to the transient interaction environment of stable water and turbulence of water,which mainly develops between the upper intertidal zone and the lower intertidal zone.